1. Field of the Invention
The invention relates to a device for calibrating and cooling a film or sheet of plastic which comprises at least two cooling or calibrating rolls, the rolls being followed by a downstream cooling zone, and also relates to a method for this.
2. Description of Prior Art
Known calibrating and cooling devices comprise, for example, a 3-roll polishing stack with a polishing roll and two cooling rolls. It operates as follows: the film runs out of the slot die into the nip between the polishing roll and the first cooling roll and is polished there. Subsequently, it runs further on the circumference of the second roll (first cooling roll)—cooling of the first side of the film—referred to as single-sided cooling. Then it runs into the 2nd nip between the second roll and the third roll and is sometimes smoothed there again. After that, it runs on the circumference of the third roll (second cooling roll)—cooling of the second side of the film—here once again single-sided cooling.
Therefore, there is the problem that, depending on the speed of the film, the thickness of the film and the geometry of the rolls (diameter), the film is intensely cooled on one side on the first cooling roll—sometimes beyond half the film thickness—so that in any event stresses may be frozen in the film and possibly lead to warpage of the end product during later processing. This phenomenon is repeated, to a rather lesser extent, on the second side. The polishing stack is therefore designed for a specific throughput and/or product range and can only be changed or corrected later with great effort. It is only possible to a limited extent to increase the output. A high cooling rate of the film, and consequently a desired high level of transparency of the film, is achieved only at a narrowly defined operating point.
There are also 3-roll polishing stacks of this type with an adjustable 3rd roll, the second cooling roll being arranged rotatably about the first cooling roll (usual angle of rotation about 130°).
The film once again runs out of the slot die into the nip between the polishing roll and the first cooling roll and is polished there. Subsequently, it runs further on the circumference of the second roll (first cooling roll)—cooling of the first side of the film—single-sided cooling. In this case, the dwell time, and consequently the cooling time, of the film on the roll can be set within certain limits by the position of the 2nd cooling roll with respect to the first cooling roll. Then it runs into the 2nd nip between the second roll and the third roll—and is sometimes smoothed there again. Subsequently, it runs on the circumference of the third roll (second cooling roll)—cooling of the second side of the film—again single-sided cooling. The contact length of the film on the 2nd cooling roll can sometimes likewise be influenced by an adjustable take-off roll. The variable arrangement of the second cooling roll, and possibly a take-off roll, makes the operating range more variable than in the case of a fixed roll arrangement. The cooling rate can be set for a greater range of thicknesses of the film. This also allows a high level of transparency of the film to be achieved over a greater range.
However, higher costs are incurred as a result of the additionally necessary mechanical equipment. A “more flexible” construction is obtained. There is a greater operating range than in the case of the conventional 3-roll polishing stack, but even with this polishing stack the operating range is restricted on account of the geometrical conditions.
Also known are belt polishing units, comprising 2 belts lying opposite each other, in which the film runs from the slot die into a gap that is formed by two circulating belts lying opposite each other and running parallel to each other, is polished between the two belts and symmetrically cooled on both sides. The film is polished on both sides and contains little stress.
The problem with this system is that the belt polishing unit can only be designed for a very narrow working range. Changing the output is not possible on account of the mechanical conditions. The width of the belt polishing unit is restricted on account of the technical possibilities and the production of wide belts with a high surface quality of great planarity is difficult and expensive. The amount of heat to be dissipated increases in proportion to the width of the belt. The side surfaces available for pipework are constant, so it becomes disproportionately more difficult to guide the belt and absorb the nip pressures as the width of the belt increases (bending beam). Furthermore, the tensioning of the belt becomes disproportionately more difficult as the width increases—sagging of the deflecting roll. A reduction of the film width (congruence) is not possible. On account of the necessary sliding surfaces for guiding the belt and absorbing the nip pressures, the system wears very quickly.
These devices can be combined, so as to create a 3-roll polishing stack and a belt polishing unit comprising one polishing roll, two cooling rolls and one or two belts, which cool the film on the side respectively lying outward on the circumferential surface of the cooling rolls.
Here too the film runs out from the slot die into the nip between the polishing roll and the first cooling roll—and is polished there. Then it runs further on the circumference of the second roll (first cooling roll)—cooling of the first side of the film—the film is simultaneously further polished and cooled on the other side of the film by the belt lying against the outer side of the film. It runs into the 2nd nip between the second roll and the third roll—is sometimes polished there once again and then runs on the circumference of the third roll (second cooling roll)—cooling of the second side of the film—here too the cooling and smoothing of the outer side of the film takes place simultaneously by the belt lying against it.
The guiding of the belts parallel to the circumference of the rolls is more difficult than straight guidance, since the temperature-dependent changes in diameter of the roll also have to be taken into account in addition to the problems of precision production of the belts. As a result of the problems in the production and operation of the belts, here too the belt width is restricted. The polishing unit is designed for a specific throughput and/or product range and can only be changed or corrected later with great effort. It is only possible to a limited extent to increase the output. A high cooling rate of the film, and consequently a desired high level of transparency of the film, is achieved only at a narrowly defined operating point.